Method and Apparatus for Firetube Boiler and Ultra Low NOx Burner

ABSTRACT

The current invention disclose a method and apparatus for production of hot water or steam in a firetube boiler and burner, said method comprising the steps of producing a first flue gas in a first stage of a burner associated with the firetube boiler, passing at least a portion of said first flue gas through a first pass and a second pass of the boiler, wherein each of said first and second passes comprises at least one firetube; routing said portion of said first flue gas to a second stage of said burner to reduce the flame temperature and NOx emissions of said second stage of said burner; producing a second flue gas in said second stage of said burner; passing said second flue gas through a third pass of the boiler, wherein said third pass comprises at least one firetube.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a boiler and burner system for the production of hot water or steam. More particularly, this invention relates to the production of hot water or steam using a firetube boiler and burner system that are designed to produce ultra low NOx emissions and higher efficiency at the same time. The firetube boiler and the burner of this invention are specifically designed to work with each other.

2. Description of the Related Art

Boilers are widely used for the generation of hot water and steam. A conventional boiler (excluding Heat Recovery Steam Generator or HRSG) comprises a furnace in which fuel is burned, and surfaces typically in the form of steel tubes to transfer heat from the flue gas to the water. A conventional boiler has a furnace that burns a fossil fuel or, in some installations, waste fuels or biomass derived fuels. According to the website of Britannica, the first boiler with a safety valve was designed by Denis Papin of France in 1679; boilers were made and used in England by the turn of the 18th century. Most conventional boilers are classified as either firetube or watertube types. In a firetube boiler, the water surrounds the steel tubes through which hot flue gases from the furnace flow. In a watertube boiler, the water is inside the tubes with the hot flue gases circulating outside the tubes. One example of firetube boilers is Scotch. Marine firetube boilers. There have been relatively few innovations in the designs of firetube boilers in the last few decades. The recent innovations have been focused on low NOx burner technologies. However, innovations in the burners have been hampered by the lack of innovations in the firetube boilers.

NOx is a recognized air pollutant. Regulations on NOx tend to get more stringent in densely populated areas of the world. In some areas, local regulations require low NOx or even ultra low NOx emissions in the exhaust from the combustion processes of the boilers. Various low NOx and ultra low NOx burners are available in the market to meet these requirements. A review of typical NOx reduction methods can be found in the article “NOx emissions: Reduction Strategy” in “Today's Boiler” magazine Spring 2015 by Jianhui Hong. FGR (Flue gas recirculation) is a commonly used technique for NOx reduction. In one common implementation called “Induced FGR”, flue gas is drawn through a pipe or duct to the inlet of a blower and mixed with the combustion air by using the blower wheel as a mixing device. The flue gas is typically at a higher temperature than the ambient air. The introduction of flue gas into the blower can sometimes lead to condensation, corrosion, and heat damage to the burner equipment. For example, condensation on the ignition system could render it inoperable. Corrosion to the internal parts of the blower and the burner head can occur. Heat and condensation from the flue gas can damage the flame scanners, which is part of the burner management system. The heat can also transmit through the shaft of the motor and damage the motor if the shaft is not properly cooled.

According to the Perry's Chemical Engineers' Handbook (7^(th) Edition) Section 10-46, the horsepower requirement for a blower is determined by the multiplication of two factors, the volumetric flow rate through the blower in cubic feet per minute, and the blower operating pressure in inches water column. Induced FGR increases both the volumetric flow rate through the blower and the pressure drop through the burner and the boiler (hence increasing the blower operating pressure), and therefore greatly increases the horsepower requirement for the blower motor. If the flue gas does not have to go through the blower wheel, the horsepower requirement of the motor does not have to be penalized for the extra volumetric flow rate of the flue gas.

U.S. Pat. No. 5,407,347A teaches an apparatus and method for reducing NOx, CO and hydrocarbon emissions when burning gaseous fuels. The advantage of this invention is that ultra low NOx emission can be achieved at relatively low oxygen level (such as 3% dry volume basis) in the flue gas. The shortcoming of this technology; is that a large amount of FGR (up to 40% of combustion air by mass) is required to achieve <9 ppm NOx emissions. In addition, the rapid mixing design requires large pressure drops across the swirl vanes in the combustion air pathway near the burner head. Since mixing rate slows down with flow velocity, this design also has a limited turndown for ultra low NOx performance. Due to the large amount of FGR and the high pressure drop the air/FGR mixture has to overcome, a larger motor and a larger combustion air blower are required compared to some other alternative ultra low NOx burner technologies. The larger motor means higher initial capital costs, higher electricity consumption and higher noise during the burner's operation. In the state of California in particular, operators of boilers often dislike use of FGR, perhaps due to the concerns of earthquake and the additional mandatory structural inspection related to the field installation of the FGR pipe. U.S. Pat. No. 6,776,609 also discussed the motor size penalty problem in details related to the use of Induced FGR.

Another commonly used technique for ultra low NOx is called “lean premixed combustion”. U.S. Pat. No. 6,776,609 was intended to teach a method for operating a burner with FGR, but it also discussed the disadvantages of the lean premixed combustion method based on fiber matrix. It disclosed that “Alzeta Corp. of Santa Clara, Calif. sells a burner for use in food processing and other industries that utilizes only excess combustion air (no FGR) to achieve the flame dilution necessary for 9-ppm NOx emissions. A dilution level of 60% on a mass basis is required”. Using the same dilution principle, Power Flame Corporation offers a product called Nova Plus. It uses metal fiber matrix elements for the “fully premixed surface stabilized combustion”. As recent regulations in some counties of California requires 6 or 7 ppm NOx, the dilution air level may be higher than 60%, which is the level of dilution required for 9 ppm NOx.

Another technique for ultra low NOx relies on lean premixed combustion, but it does not use fiber matrix elements. Sellers Manufacturing, a subsidiary of Green Boiler Technologies, sells an S-Series boiler that generates 9 ppm NOx level using lean premixed combustion based on injection nozzles instead of fiber matrix elements. A fully premixed air/fuel mixture goes through multiple injection nozzles at relatively high velocity to resist flashback to areas upstream of these injection nozzles.

The shortcomings of the “lean premixed combustion” technique are well recognized in the combustion community: low thermal efficiency due to the very high excess air level and the resultant very high oxygen level in the flue gas (9% oxygen is typical), and the extra electricity consumption due to the extra excess air for the dilution effects. The large amount of excess air was intended to reduce the peak flame temperature by dilution effects. The extra dilution air carries additional heat into the atmosphere (wasted heat) when the exhaust is vented, and causes a reduction of thermal efficiency.

In view of the foregoing, there exists a need for an improved method and apparatus for production of hot water and steam that can produce low NOx (including ultra low NOx) emissions, low electricity consumption for the motor and high thermal efficiency at the same time.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide a method and apparatus for the production of hot water or steam in a firetube boiler and burner system that produces low NOx emissions, low electricity consumption and high thermal efficiency.

A more specific object of the present invention is to provide a method and apparatus for the production of hot water or steam in a firetube boiler and burner system that produces ultra low NOx emissions in the flue gas, low oxygen level in the flue gas which leads to higher thermal efficiency, low horsepower requirement for the blower motor for the burner.

These objects are achieved by a method of producing hot water or steam, comprising the steps of, producing a first flue gas in a first stage of a burner associated with a firetube boiler; passing at least a portion of said first flue gas through a first pass and a second pass of the boiler, wherein each of said first and second passes comprises at least one firetube; routing said portion of said first flue gas to a second stage of said burner to reduce the flame temperature and NOx emissions of said second stage of said burner; producing a second flue gas in said second stage of said burner; passing said second flue gas through a third pass of the boiler, wherein said third pass comprises at least one firetube.

These objects are achieved by an apparatus for producing hot water or steam, comprising a firetube boiler and a burner, said firetube boiler comprising a shell substantially cylindrical in shape, the shell having a front end, a back end, and a water entry port; a plurality of firetubes disposed in the shell and substantially extending the length of the shell between the front end and the back end, said firetubes form a first pass, a second pass and a third pass in the boiler, wherein said first pass comprises at least one of said firetubes and allows flue gas to flow in the direction from the front end to the back end; said second pass comprises at least one of said firetubes and allows flue gas to flow in the direction from the back end to the front end; said third pass comprises at least one of said firetubes and allows flue gas to flow in the direction from the front end to the back end; and said burner located in the vicinity of the front end of said firetube boiler comprising of a first stage that produces a first flue gas; a second stage that produces a second flue gas; wherein the first flue gas goes through said first and second passes of said boiler, and is routed to the second stage of the burner to reduce the flame temperature and NOx emissions of said second stage; said second stage produces a second flue gas that goes through said third pass of said boiler.

Additional objects and features of the invention will appear from the following description from which the preferred embodiments are set forth in detail in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an apparatus for producing steam in accordance with the present invention.

FIG. 2 is a schematic view of an alternative apparatus for producing steam in accordance with the present invention.

FIG. 3 is a schematic view of yet an alternative apparatus for producing steam in accordance with the present invention.

FIG. 4 is a schematic view of another alternative embodiment of the apparatus for producing steam in accordance with the present invention.

FIG. 5 is a sectional view of the boiler in FIG. 4 taken along lines A-A.

Identical reference numerals throughout the figures identify common elements.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purpose of this invention, a firetube shall mean a tube that allows a flue gas to flow inside, and shall also include a tube that allows combustion to occur inside, even if combustion may be in progress in the majority of the length of the tube. For example, a Morrison tube, or a furnace tube, whether corrugated or not, is considered a firetube for the purpose of this invention. No distinction is made between a furnace and a firetube.

A burner shall mean a device to produce one or more flames in the firetube boiler of the current invention in a controlled manner, taking inputs from at least one fuel source and an oxidizer source such as air. The two stages of the burner disclosed in this invention could arguably be referred to as two separate burners by anyone skilled in the art, at least for some of the embodiments. But such change of nomenclature does not create a new invention outside the scope of the current invention.

FIG. 1 shows a schematic view of an apparatus for the current invention. A boiler 5 has a cylindrical shell 30, which is welded to tubesheet 31 and tubesheet 32 to form a pressure vessel 40. The boiler has a front end 6 near tubesheet 31, and a back end 7 near tubesheet 32. Feed water is fed into the boiler through water inlet 42. When necessary, water can be drained through drain outlet 43. Steam is collected in the vapor space within the pressure vessel 40 and above the water level 41, and discharged through steam outlet 48 when pressure is higher than a desired pressure setpoint.

A burner 10 has a first stage 11 and a second stage 12. Each of these two stages of the burner 10 is supplied with fuel and combustion air in a proper air/fuel ratio so that combustion can be sustained, a means of ignition to start combustion, and a means for flame monitoring to ensure safety. For clarity and simplicity of illustration, the details of these two stages of the burner are omitted in FIG. 1. The combustion air to these two stages of the burner may be supplied by two separate blowers, or may preferably be supplied by a single blower. The fuel supplied to these two stages of the burner may be taken from the same source of fuel, or two separate sources of fuel. Air and fuel supplies to these two stages (11 and 12) may be modulated or shutoff. Means of ignition may include a spark, a nearby flame, or a solid surface at a temperature higher than the auto-ignition of the combustible mixture and any other method. Means for flame monitoring may include UV, IR, flame rod and any other method.

The first stage 11 of the burner 10 produces a first flue gas in firetube 33, which is the first pass of the boiler. The first flue gas flows in the first pass in the direction from the front end 6 to the back end 7. The first flue gas exits from the first pass into a chamber 50 affixed to the back end 7, then goes through firetube 35, which is the second pass of the boiler in the direction from the back end to the front end, and discharges into a chamber 20 affixed to the front end 6. The second stage 12 is disposed in chamber 20 to produce a flame. The first flue gas in chamber 20 is mixed with the combustible from the second stage 12 and functions to reduce the flame temperature of the second stage 12, and thus reduce NOx emissions from the second stage 12 of the burner 10. Using flue gas to reduce NOx emissions is well understood in the combustion community. The advantages of the current invention include three aspects: firstly the flue gas does not go through the wheel of any blower, thus reducing the motor horsepower requirement and electricity consumption of the motor; secondly the use of flue gas helps reduce the oxygen level in the flue gas while maintaining the desired low NOx levels, thus improves the thermal efficiency of the boiler; thirdly there is no need for an external FGR pipe. Elimination of the external FGR pipe is advantageous as is discussed in the section of “Description of Related Art”.

The second stage produces a second flue gas in firetube 38, which is the third pass of the boiler. The second flue gas exits the third pass and discharges into a chamber 60 affixed to the back end 7, and goes through a group of firetubes 39, which are the fourth pass of the boiler. The second flue gas exits the fourth pass, and discharges into a flue gas collection chamber 70 affixed to the front end 6, and is vented out of the boiler through flue gas outlet 80.

The stages 11 and 12 of the burner 10 are located in the vicinity of the front end 6. The observation ports 52 and 62 are located in the vicinity of the back end 7. Ports 52 and 62 allow manual observation of the flames in the first pass and the third pass, respectively. For simplicity, insulation and refractory materials as commonly used for boilers are not shown in any figures in this invention.

It is well known that burners can be classified as premix type or diffusion type (also known as non-premix type), depending on whether the fuel and air is mixed well before combustion is initiated. Each of the stages 11 and 12 in FIG. 1 can be either a premix type or diffusion type. For the purpose of this disclosure, the words “premix” and “premixed” are interchangeable.

Burner 10 can be operated in several modes. In a first mode, the first stage 11 and second stage 12 are both in operation, converting fuel and air into flue gas and generating heat. This is the normal mode of operation where low and ultra low NOx emissions are desired. In a second mode, the first stage 11 is in operation, and the second stage 12 is turned off. In this mode of operation, the flue gas from the first stage 11 still goes through the first, second, third and fourth passes of the boiler, but the fuel supply to the second stage is turned off. The combustion air supply to the second stage of the burner is kept on but modulated to a minimal flow rate, just to prevent the flue gas from the second pass of the boiler to back flow in the combustion air duct leading to the second stage and cause damages. This is the mode of operation when extremely high turndown is desired for the boiler. Caution should be used to limit the maximum turndown to avoid condensation in the firetubes, if the boiler is not designed as a condensing boiler. In a third mode of operation, the first stage 11 is turned off, and the second stage 12 is in operation. To prevent back flow in the first and second passes, the fuel supply to the first stage is shut off, and certain amount of combustion air is supplied to the first stage. This may be an acceptable mode of operation when the requirement for NOx emissions is not stringent, and the full capacity of the boiler is not required. This mode of operation does not take full advantage of the current invention, and is generally not recommended.

The third and fourth passes (firetube 38 and firetubes 39) of the boiler in FIG. 1, correspond to the first and second passes of a two-pass boiler, if the first stage 11 and firetubes 33 and 35 are thought of as a dedicated flue gas generator. The third and fourth passes can arguably be referred by anyone skilled in the art as the first and second passes of the second stage since fuel and air from the second stage is indeed making a first and second passes through the boiler. However, during normal operation, the atoms of flue gas from the first stage of the burner have already made two passes (firetubes 33 and 35) through the boiler when they go through firetubes 38 and 39, and therefore making a third and fourth passes through the boiler. Calling firetubes 38 and 39 as first and second passes for the second stage of the burner by anyone skilled in the art is simply a choice of nomenclature, and does not create a new invention outside the scope of this invention.

It is common in the firetube boiler industries to have one-pass, two-pass, three-pass and four-pass conventional firetube boilers. Additional passes can be added to the boiler in FIG. 1. For example, a fifth pass could be added to the boiler in FIG. 1 allowing flue gas from the fourth pass to flow in the direction from front end to the back end, and the exhaust outlet 80 would move to the back end of the boiler, similar to a 3-pass boiler. Similarly, a sixth pass could be added, and the exhaust outlet 80 would stay at the front end of the boiler, similar to a 4-pass boiler.

It is common in the firetube boiler industries to have dry back and wet back designs. FIG. 1 shows a dry back design. But a wet back design could be easily implemented for the current invention by anyone skilled in the art.

Even though the first pass, the second pass and the third pass in FIG. 1 all comprise a single firetube, it should be obvious to anyone skilled in the art to recognize that any one of these passes could comprise multiple firetubes. For example, second pass could have multiple firetubes 35, as shown in FIG. 2. These firetubes 35 of second pass could be distributed around the perimeter of firetube 38 to allow the flue gas from the first stage to enter firetube 38 more evenly. FIG. 2 shows a schematic view of this alternative embodiment. Notice how chamber 50 is modified from FIG. 1 to FIG. 2 to allow firetubes 35 to be distributed around firetube 38, and how firetube 38 is extended to penetrate through the chamber 50 and discharge the second flue gas from the third pass into chamber 60.

FIG. 3 shows an alternative embodiment of the current invention. It is similar to FIG. 1 in general except that the first flue gas from the first stage passes through first pass (firetube 33) and second pass (firetube 35) and is routed to through a pipe 20A to a chamber 20. Chamber 20 is designed to deliver the first flue gas to the perimeter of the second stage 12 of the burner 10, so that the first flue gas can participate in the combustion of the second stage as early as possible to suppress formation of thermal NOx. Further details of the burner 10 is provided in FIG. 3. A single blower 1 supplies combustion air to both stages 11 and 12 of burner 10. Combustion air is drawn in from inlet 2 by blower 1, goes through air duct 3A and 3B to stage 11 and 12 of burner 10, respectively. Natural gas is supplied from a single source (not shown) through fuel lines 4A and 4B to stage 11 and 12 of burner 10, respectively. Fuel flows through 4A and 4B are modulated by modulation valves (not shown) and can be shut off by safety shutoff valves (not shown). Combustion air flow through 3A and 3B are modulated by two louver box dampers (not shown) and a VFD (variable frequency drive, not shown) on the motor of the blower. Stage 11 and stage 12 are equipped with independent means for ignition and flame monitoring systems (not shown).

In FIGS. 1-3, the first stage 11 is generally rated for a smaller fraction of the total heat input of the burner 10 than the second stage 12. The heat input of the first stage 11 as a percentage of the total heat input of burner 10 depends on the need of flue gas for the second stage. The more flue gas is needed for NOx suppression in the second stage, the larger fraction the first stage needs to be. There is an upper limit on how much flue gas the second stage can take before the second stage becomes unstable or CO emissions from the second stage becomes unacceptably high, which is not desirable since CO is also a regulated air pollutant. In general, the first stage 11 should account for 10-35% of the total heat input of the burner 10, and the second stage accounts for the balance.

FIG. 4 shows a schematic view of yet another embodiment for the current invention. This embodiment utilizes the “premixed combustion” technique, but overcomes the shortcomings (high excess air and low efficiency) of “lean premix combustion” used by conventional ultra low NOx burners. A blower 1 supplies the combustion air to the burner 10. Combustion air is drawn in from inlet 2, and goes through air duct 3C and 3D. Air duct 3C supplies the majority of the combustion air to a mixing chamber 9. A gaseous fuel is supplied through fuel line 4C to mixing chamber 9. The air/fuel ratio in the mixing chamber 9 is maintained so that the oxygen level in the flue gas out of the exhaust outlet 80 is at a desired level such as 3% dry volume based. The premixed air/fuel mixture from mixing chamber 9 enters a distribution manifold 8 and is distributed to 6 burner nozzles that are fluidically communicating with manifold 8, said nozzles including nozzle 21 and nozzles 22 (a group of five nozzles substantially aligned to a group of firetubes 38, see FIG. 5). The first stage 11 of the burner 10 comprises nozzle 21. The second stage 12 of the burner 10 comprises nozzles 22. Since 100% of the fuel of the burner is supplied through fuel line 4C, the heat input split between the first and second stages are determined by number of nozzles in the second stage, and the exit areas of nozzle 21 and nozzles 22, and to some extent by the additional back pressure caused by the first and second pass of the boiler. In general, the first stage should account for 10-35% of the total heat input of the burner, while the second stage accounts for the balance. In this particular embodiment in FIG. 4, one stage of the burner cannot be turned off while keeping the other stage on.

A small fraction of the combustion air supplied by blower 1 goes through air duct 3D, and goes into the first stage 11 of the burner 10, to increase the air/fuel ratio of the premixed mixture of air and fuel so that the oxygen level in the first flue gas is at a higher level such as 7-10%. The first stage of the burner utilizes the lean premix technique commonly used in the conventional ultra low NOx burners. The high excess air was necessary for lowering the flame temperature, which in turn suppress NOx formation. The higher oxygen level in the first flue gas allows the NOx emissions in the first flue gas to reach ultra low NOx levels. However, this higher level of oxygen in the first flue gas does not result in lower efficiency of the boiler, because at least some of the oxygen in the first flue gas will be consumed in the second stage of the burner. The oxygen level in the second flue gas is maintained at lower levels such as 1-6% dry volume based, preferably at 1-3% dry volume based.

The first stage 11 produces the first flue gas in firetube 33, which is the first pass of the boiler. The first flue gas then goes through an U-shaped tube 50A and passes through firetube 35, which is the second pass of the boiler. The first flue gas is then discharged into chamber 20. The nozzles 22 of the second stage 12 are disposed in chamber 20, so that the first flue gas from the second pass can be distributed substantially evenly to the five nozzles 22 to help suppress the formation of thermal NOx of the second stage. Tube 50A is connected to a bypass tube 39. Both tube 50A and bypass tube 39 could be further equipped with valves so that a portion of the first flue gas could be directed through the bypass tube 39 and discharged into chamber 70, instead of going to second pass of the boiler. When bypass tube 39 is shut off completely, all the flue gas exiting the first pass is directed though tube 50A to the second pass.

The first stage 11 is ignited by an ignitor through port 26 to produce a flame in chamber 24. This flame can start outside the firetube 33, and ends typically somewhere within firetube 33. Chamber 24 is disposed in chamber 20. Chamber 24 has small ports communicating with chamber 20 so that a flame associated with nozzle 21 in chamber 24 can ignite the adjacent nozzles 22. Nozzles 22 are disposed close enough to each other so that cross-ignition can occur. This cross-ignition avoid the need for an ignitor for each nozzle within the second stage 12.

The flames associated with the second stage 12 start within chamber 20 but outside of firetubes 38, and typically ends somewhere within firetubes 38. Firetubes 38 are the third pass of the boiler. The flue gas from the second stage exits from the third pass into a flue gas collection chamber 70, and is routed through a flue gas outlet 80 to other devices such as a chimney or an economizer.

Chamber 24 directs the flame from the first stage into firetube 33, and creates a pressure drop between the front end of firetube 33 and the front end of firetube 35. This pressure drop is necessary to drive the flue gas to travel from right to left in firetube 35 and into fire chamber 20. Chamber 24 may have small openings to allow detection of the flame within chamber 24 using a scanner mounted on port 28.

We now turn our attention to the production of steam in FIG. 4. The firetubes 33, 35 and 38 are supported on both ends by tubesheets 31 and 32. The vessel shell 30 and tubesheets 31 and 32 together form a boiler vessel 40. Feed water goes into the boiler vessel 40 through inlet 42. Water level 41 is maintained above the top of firetube 38 to avoid heat damage to the firetubes. Some heat in the flue gas is transferred through the firetubes 33, 35 and 38 to the water in vessel 40 to produce steam. Steam collects in the space within the vessel 40 and above water line 41, and is released out of the boiler through steam outlet 48. If necessary, water in the vessel 40 can be drained through drain port 43.

As is well understood in the boiler industry, if hot water production is desired instead of steam, steam outlet 48 in FIGS. 1-4 would be replaced by a hot water outlet located at a proper location on the shell 30.

The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. In other instances, well known devices are shown in block diagram form in order to avoid unnecessary distraction from the underlying invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, the thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents. 

What is claimed is:
 1. A method of producing hot water or steam, comprising the steps of, producing a first flue gas in a first stage of a burner associated with a firetube boiler, passing at least a portion of said first flue gas through a first pass and a second pass of the boiler, wherein each of said first and second passes comprises at least one firetube; routing said portion of said first flue gas to a second stage of said burner to reduce the flame temperature and NOx emissions of said second stage of said burner; producing a second flue gas in said second stage of said burner; passing said second flue gas through a third pass of the boiler, wherein said third pass comprises at least one firetube.
 2. The method as described in claim 1 further comprises a step of passing said second flue gas through a fourth pass of the boiler, wherein said fourth pass comprises a plurality of firetubes.
 3. The method as described in claim 2 further comprises a step of passing said second flue gas through a fifth pass of the boiler, wherein said fifth pass comprises a plurality of firetubes.
 4. The method as described in claim 3 further comprises a step of passing said second flue gas through a sixth pass of the boiler, wherein said sixth pass comprises a plurality of firetubes.
 5. The method as described in claim 1 wherein the first stage and the second stage of the burner are supplied with combustion air from a single blower.
 6. An apparatus for producing hot water or steam, said apparatus comprising a firetube boiler comprising a shell substantially cylindrical in shape, the shell having a front end, a back end, and a water entry port; a plurality of firetubes disposed in the shell and substantially extending the length of the shell between the front end and the back end, said firetubes form a first pass, a second pass and a third pass in the boiler, wherein said first pass comprises at least one of said firetubes and allows flue gas to flow in the direction from the front end to the back end; said second pass comprises at least one of said firetubes and allows flue gas to flow in the direction from the back end to the front end; said third pass comprises at least one of said firetubes and allows flue gas to flow in the direction from the front end to the back end; and A burner affixed to the front end of said firetube boiler comprising a first stage that produces a first flue gas; a second stage that produce a second flue gas; wherein at least a portion of said first flue gas goes through said first and second passes of said boiler, and is routed to the second stage of the burner to reduce the flame temperature and NOx emissions of said second stage; said second flue gas goes through said third pass of said boiler.
 7. The apparatus as described in claim 6 wherein said firetube boiler further comprises a fourth pass that comprises a plurality of firetubes and allows flue gas to flow in the direction from the back end to the front end.
 8. The apparatus as described in claim 7 wherein said firetube boiler further comprises a fifth pass that comprises a plurality of firetubes and allows flue gas to flow in the direction from the front end to the back end.
 9. The apparatus as described in claim 8 wherein said firetube boiler further comprises a sixth pass that comprises a plurality of firetubes and allows flue gas to flow in the direction from the back end to the front end.
 10. The apparatus as described in claim 6 wherein the first stage and second stage of the burner are supplied with combustion air from a single blower.
 11. The apparatus as described in claim 6 wherein the first stage and second stage of the burner use non-premix type combustion.
 12. The apparatus as described in claim 6 wherein the first stage and second stage of the burner use premix type combustion.
 13. The apparatus as described in claim 6 wherein the firetube boiler is a Scotch Marine dry back design.
 14. The apparatus as described in claim 6 wherein the firetube boiler is a Scotch Marine wet back design.
 15. The apparatus as described in claim 6 wherein the first pass comprises one firetube, the second pass comprises one firetube disposed at the center of a circle, and the third pass comprises multiple firetubes disposed on said circle, wherein the first flue gas exiting the second pass is distributed evenly to the third pass.
 16. The apparatus as described in claim 10 wherein the first stage accounts for 10%-35% of the heat release of the burner, and the second stage accounts for 65-90% of the heat release of the burner, when both stages are in operation.
 17. The apparatus as described in claim 10 wherein the first stage and second stage of the burner use non-premix type combustion.
 18. The apparatus as described in claim 10 wherein the first stage and second stage of the burner use premix type combustion.
 19. The apparatus as described in claim 16 wherein the first stage and second stage of the burner use non-premix type combustion.
 20. The apparatus as described in claim 16 wherein the first stage and second stage of the burner use premix type combustion.
 21. The apparatus as described in claim 20 wherein the first stage of the burner is run at a higher oxygen level in the flue gas than the second stage.
 22. The apparatus as described in claim 21 wherein the first stage of the burner is run at 7-10% oxygen level in the first flue gas dry volume based, and the second stage is run at 1-6% oxygen level in the second flue gas dry volume based.
 23. The apparatus as described in claim 22 wherein the second stage is run at 1-3% oxygen level in the second flue gas dry volume based. 